That is unfortunate. The entire site is down so this is a different situation from before where the files were available, but just not via SSL.
I have a copy of the file you need:
micronucleus-cli-2.5-azd1b-x86_64-mingw32.zip (1.1 MB)
You can follow the "2. Add the Downloaded File to the Boards Manager Cache" instructions from my previous reply, except using the file downloaded from the attachment.
You should be very wary of running an executable downloaded from a random forum post attachment. You can verify the file above is the same as the one previously available from azduino.com by getting the SHA-256 checksum of the downloaded file by opening a PowerShell terminal in the folder where you downloaded the ZIP file and then running this command:
Get-FileHash -Path micronucleus-cli-2.5-azd1b-x86_64-mingw32.zip -Algorithm SHA256
After that, compare it to the checksum for the file you can find by opening the ATTinyCore "Additional Boards Manager URLs" URL in your browser and then searching the page for "micronucleus-cli-2.5-azd1b-x86_64-mingw32.zip". The URL is available from the ATTinyCore installation instructions:
The Arduino IDE Boards Manager actually compares the checksum of the file from the Boards Manager cache against the file in the package index and only uses the file if the checksums match.
I checked on my Linux and macOS machines but unfortunately I didn't have the file cached on those machines so we only have the Windows file as of now and I didn't find another source for the files. If anyone needs to install ATTinyCore on other operating systems, you can post on the forum and hope one of the other forum helpers can provide the file.